Coating composition for metals



@OATING COMPU'SITION FOR METALS David K. Lesser, Wilmington, DeL, assign-r to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 7, 1954, Serial No. 435,082

1 Claim. (Cl. 148-6.16)

This invention relates to a coating composition for metals and more particularly to a coating composition which has improved adhesion to metals, the ability to inhibit the corrosion of metals and wherein a mixture of ethyl cellulose and polyvinyl butyral resin is the filmforming ingredient.

The chemical treatment of metal surfaces to render them resistant to corrosion and provide a suitable surface to which a protective coating will adhere has been the object of considerable investigation. Such chemical treatment is commonly accomplished by the use of wash primers which are organic coatings applied in very thin films, in the order of 0.3 mil, as a pretreatment conditioner for subsequent painting. In view of the thinness of the coating, it is quite important that the film produced is uniform and continuous. If a nonuniform and discontinuous film is produced due to contamination or lack of good film properties for any reason, the composition has little value or at best is inferior as a wash primer.

Three of the most difficult properties to obtain in organic coatings for metal are adequate adhesion, impact resistance, and prevention of under film corrosion of the metal. Numerous treatments and coatings have been developed in the past in an endeavor to obtain these desired properties. They include phosphoric acid treatment (e. g. Bonderizing), zinc chromate primers, red

ead primers, and phenolic resin primers.

Wash primers, which represent both a metal treatment and an organic coating, are one of the most recent methods for improving metal coatings. The earliest wash primer, a two-package system, was called the WP-l type. This system employed a dispersion of zinc chromate, polyvinyl butyral, and solvents inone. package, and a phosphoric acid diluent in the other. This wash primer had poor can stability (about eight hours), was expensive, and its film was prone to blister under conditions of high humidity. An improved wash primer called the chromicacid type was later developed. This primer consisted of an alcoholic solution of polyvinyl butyral reacted at elevated temperature with phosphoric and chromic acids. This later chromic acid wash primer has advantages over WP-l primers in that can stability was improved (over one year) as well as resistance to humidity. The major disadvantages to the chromic acid wash primers now commerically available are the high cost of raw materials, the hazardous chemicals used in their manufacture, and their susceptibility to contamination which usually results in poor performance.

With these disadvantages in view, it is, therefore, a primary object of this invention to provide an improved corrosion inhibiting coating or wash primer of the chromic-acid type for metals including steel, aluminum, and the like wherein relatively low cost raw materials are utilized which are readily available and which may be reacted at room temperature with satisfactory results to produce a wash primer having good properties in respect to impact resistance, scratch adhesion of sub- 2,798,016 C6 Patented July 2, 1957 sequent coatings and resistance to contamination, which properties additionally contribute to improving the corrosion resistance of the wash primer.

Other objects of the invention will appear hereinafter, the novel features and combinations being, set forth in the appended claim.

According to this invention, there is provided a coating composition, characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals, comprising thereaction product of a solution of chromium trioxide, phosphoric acid and a mixture of ethyl cellulose andpolyvinyl butyral resin dissolved in organic solvent.

The following examples are illustrative of the invention wherein all parts given are parts by weight.

Example 1 A first solution was prepared by adding 4.6 parts of chromium trioxide to' 9.3 parts of water. A second solution was prepared by'adding 10.9 parts of orthophosphoric acidto 98.6 parts of acetone. Still a third solution was prepared by adding 98.5 parts of N-10 ethyl cellulose and 10.9 parts of Vinylite XYHL to 622.0 parts of 3A ethyl alcohol. All solutions were prepared with continuousagitation. The first solution was then slowly added to the second solution with continued agitation. The mixture of the first solution and the second solution was then slowly added to the third solution with mild and continued agitation and the mixture thus formed was allowed to react for 40 minutes at room temperature. Then parts of n-butanolwas added to the reaction mixture and agitation was continued for approximately 15' minutes. green color, arsolids content of 12%, and a viscosity of 56 seconds (Ford No. 4 cup).

Example 2 The same solutions shown in. Example 1 were used in the same proportions and mixed in the same way. However, the. resin solution was heated to 50 C., then the solution of acids was added. The mixture was held at this temperature for 25 minutes after which it was cooled. When the temperature of the reaction mixture reached 35 C., 145 parts of n-butanol was added and the resulting composition was cooled to room temperature. This composition hada medium green color, a solids content of 12%, and a viscosity of 41 seconds (Ford No. 4 cup).

Compositions in accordance with this invention which were prepared as set forth in the foregoing examples were applied to metal panels. The panels tested were aluminum (3S) and cold rolled steel. The panels were first thoroughly cleaned in solvent, then one-half of each panel was dipped in the Wash. primer of this invention using a dip coater adjusted to provide a 0.3-0.5 mil dry film. One setof'panels was topcoated the same day as dipping, another set of panels was topcoated after aging one week, and still another set of panels was topcoated after baking 15 minutes as 250 F. After the designated drying schedule, the entire panel was sprayed with three coats of nitrocellulose lacquer.

Impactresistance testsand scratch adhesion tests carried out on the various sets of panels showed generally an improvement in impact resistance and in scratch adhesion of the film applied over the wash primer of this invention in comparison with the film applied over the bare metal. Thus, in addition to the known corrosion resistance imparted by Wash primers, wherein phosphoric acid and chromic acid have been disclosed as reacting with a metal surface to deposit a tightly adhering and insoluble layer of metal salt, there is provided an excellent film-forming constituent which is ethyl cellulose The resulting composition had a medium and polyvinyl butyral bound in some manner to form a tightly adhering complex of the whole composition to metals. during the preparation of the compositions of 'this'invention arenotknown. Additionally, the exact mechanism by which the compositions of this invention are bound to metals is not known. physically that the ethyl cellulose-polyvinyl butyral film or complex thereof is smooth, uniform, continuous and adhesive which is a highly important requisite for av corrosion inhibiting coating which also serves as a ground orv base coat for painting.

It is evident that there are several factors which: will influence conditions for the most satisfactory operation of this invention, the actual limits of which can be determined only by a detailed study of each set: of materials to be utilized in preparing the compositions and the types of metals to which the compositions are to be applied. For example, the following factors :will: affect thesuitability of the compositions of this invention for certain specific application: composition of solvent blend,

The complex chemical reactions which occur However, it has been observed water content, ratio of'chromicacid to'phosphoric acid and acid concentration, optimum reaction time and temperaturc, type and grade of ethyl cellulose and polyvinyl butyral resin and type of 'metal' to which the composition is to be applied.

Generally, however, it will be found that compositions prepared in proportions and using ingredients: approximating those set forth in the examples will usuallygive satisfactory results for general application to certain metals. In this respect, although 3A (methanol denatured) alcohol is preferred as set forth in the examples, 2B (benzene denatured) or other alcohol or other suitable organic solvent may be used: Although N 10 (46.8-48.5% ethoxyl content and a viscosity of -10 centipoises in 5% concentration solution) ethyl cellu-. lose is preferred .as set forth .the'examples, vorganosoluble ethyl cellulose of other ethoxyl content and viscosity may be used within practical limits. Similarly,

although Vinylite 'XYHL (vinyl content 54.4, butyraldehyde 38.3, acetate 0.3 and hydroxyl 7.0% with intrinsic viscosity of 0.81) polyvinyl butyral resin is preferred as set forth in the examples, other organosoluble vinyl butyral resins may be used within practical limits in combination with organo-soluble ethyl cellulose for obtaining the reaction product and coating composition of this invention. It has been found from test results that a 9:1 ratio of ethyl cellulose2polyvinyl butyral resin reacted at room temperature appears to approach a maximum in performance relative to impact resistance and scratch adhesion. Water is necessary as the solvent for chromium trioxide in order to avoid the spontaneous combustion which may occur when chromium trioxide is mixed directly with alcohol. Acetone is preferred for preparing the phosphoric acid solution, although other similar solvents may be used such as methyl ethyl ketone. The amount of chromium trioxide in the reaction mixture should be less than the amount of phosphoric acid present and, based on 1000 parts by weight of the total reaction solution, the chromium trioxide, phosphoric acid, ethyl cellulose, and polyvinyl butyral resin may be varied Within the following ranges: 0.5 to 5; 1 to 20; 10 to 200; and 10 to 200, respectively. The concentration of the reaction product of this invention based on the total weight of the coating composition may be varied from about 3. to about by weight. Theprimer of this invention is usually used without the addition of pigsuch as by dipping, spraying, and brushing. However,

the primer coating composition of thisinvention may be applied using the above conventional methods of application and suitably dried at room or elevated temperature. Additionally, the compositions may be applied to a metal base by dipping or immersion with or without residence time and then suitably dried.

Certain uses for the coating compositions of this invention reflect its advantages in giving asmooth, uniform, continuous and adhesive film and include more particularly: automobile finishes in place of or supplementing Bonderizing (phosphoric acid treatment); pretreatment conditioner or pigmented primer for baking systems such as refrigerators or steel cabinets; pretreatment on casting or steel moldings where corrosion would be prevented during aging and undercoat provided; coating material for sheet steel to prevent corrosion in transit and storage and undercoat providedforany desired subsequent finishing; and coating material for aluminum foil door frames, house siding, trailers, boats and airplanes Where improved adhesion and corrosion resistance are desired.

What I claim and desire to protect by Letters Patent is:

: A coating composition, characterized by improved adhesion to metals and the ability to inhibit the corrosion of metals, consisting essentially of the reaction product of a solution of 0.5 to 5 parts by weight of chromium trioxide, 1m 20 parts'by weight of phosphoric acid, 10 to 200 parts by weight of organo-soluble ethyl cellulose and 10 to 200 parts by weight of polyvinyl butyral resin dissolved in an organic solvent solution consisting of a mixture of lower aliphatic alcohols, ketones and Water, said alcohols having 1 to 4 carbon atoms, said ketones being solvents for phosphoric acid solution, the amount of said alcohols being sufficient to dissolve said ethyl cellulose and polyvinyl butyral resin, the amount of said ketones being sufficient to dissolve said phosphoric acid solution, and the amount of said Water being suflicient to dissolve said chromium trioxide, said chromium trioxide being added to said solution in the form of an aqueous solution and in an amount less than the amount of said phosphoric acid, the concentration of said reaction product being from 3% to 20% by weight based on the total weight of said coating composition, and said coating composition being prepared at temperatures up to about C.

References Cited in the file of this patent UNITED STATES PATENTS 1,254,263 Oeschger Jan. 22, 1918 1,514,494 Williams Nov. 4, 1924 2,293,558 Overholt Aug. 12, 1942 2,412,543 Tanner Dec. 10, 1946 2,469,318 Swan May 3, 1949 2,525,107 Whiting et al. Oct. 10, 1950 

